The stopping process may be initiated in order to avoid overload situations for a wind turbine, for example, if the electric generator of the wind turbine suddenly is disconnected from the utility grid.
Previously, wind turbine blades of a wind turbine have been controlled during a stopping process by using pre-established values for the pitch velocity until the blades are fully pitched out of the wind.
The pitch control systems mainly comprise mechanical and hydraulic components such as hydraulic actuators to turn the wind turbine blades in or out of the wind around their longitudinal axis.
A problem with the prior art is that the values for the pitch velocity have to be picked conservatively in order to ensure that any part of the wind turbine, and especially the blades, do not experience an overload situation from the controlling during the stopping process. However, if the stopping process is to slow, the generator and gear means of the wind turbine may suffer damage.
A related problem is that the wind turbine, and especially the blades, have to be dimensioned quite robust in order to survive any force during the stopping process in which the process is prolonged by the low pitch velocity.
A further problem is the fact that the rotor may get out of control, i.e., accelerate to fatal speeds due to the low pitch velocity.
An object of the invention is to establish a technique without the above-mentioned disadvantages. Especially, it is an object to establish solutions which are optimal in connection with the stopping process of wind turbine rotors and especially rotors of large modern wind turbines without causing damage to the wind turbines.